A common view in philosophy is that counterfactual conditionals with impossible antecedents are all vacuously true. But I’m very worried about that view. There are several counterfactuals with impossible antecedents that seem false. Here are some examples.

Counterfactuals involving Metaphyical Claims
Consider Metaphysical Nihilism, the thesis that there are no composite objects (just simples). According to Nihilism, composition doesn’t occur. Suppose you think that metaphysical facts about composition are necessarily true. Under that assumption if Nihilism is false, then (1) is a false counterfactual with an impossible antecedent. If Nihilism is true, then (2) is a false counterfactual with an impossible antecedent.

(1) If Nihilism were true, then composition would occur. (seems false)

(2) If Unrestricted Composition were true, then there would be no composition.

If you think that Metaphysical Facts about composition are not necessarily true, then you should think that (3) is a false counterfactual with an impossible antecedent.

(3) If Nihilism were necessarily true, then composition would occur. (seems false)

Any way you go on the answer to the Special Composition Question, and any way you go concerning the modal status of composition facts – we can generate a false counterfactual with an impossible antecedent.

Counterfactuals Involving Math and Geometry
Here’s another fun one…

(4) If triangles had exactly four sides, then the sum of the angles of a triangle would equal 30 degrees.

(False, right?)

However…
I also have some sympathy toward the view that counterfactuals with impossible antecedents are vacuosly true. I can post about that later. I can also post about why I think this matters.